The Gravity Field and Steady-State Ocean Circulation Explorer, launched in 2009, collected data to map Earth's gravity with unrivaled precision. While we generally think of gravity as constant (an assumption well known to Physics 101 students), the force varies regionally due to uneven distribution of material deep within the earth. The GOCE satellite was tasked with measuring this variation to produce a detailed gravity map, or geoid:

What researchers didn't expect was that the GOCE satellite would capture a real-time change in gravity. But the 2011 earthquake, the fifth-strongest ever recorded, shifted rock formations several kilometers below earth's surface, and changed the shape of the sea bed, altering the gravitational pull off Japan's coast. The quake was so powerful, instruments on the satellite even recorded sound waves emanating from the tectonic movement.

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In the image at the top of this post, we see areas of reduced (blue) and increased (yellow) gravitational pull around the epicenter of the earthquake (indicated by the yellow and white "beach ball"). Compare this to the gravity map completed a year before the earthquake, where Japan sits in a relatively uniform gravitational plane: